SpaceX, NASA realign launch and EVA scenarios

SpaceX managers have confirmed a second attempt to launch their Falcon 9 v1.1 with the CRS-3 Dragon spacecraft to the International Space Station (ISS) will take place on Friday, with Saturday as a back-up opportunity. Meanwhile, Station managers have created an EVA schedule to work around when Dragon finally sets sail. Currently, EVA-26 – which will replace a failed Multiplexer/Demultiplexer (MDM) – will now likely take place on April 23.

At the time, the issue that put pay to the launch was only classed as a helium system leak on the first stage of the Falcon 9 v1.1. However, SpaceX has since updated the specifics of the problem, confirming what NSF sources had noted during the immediate aftermath of the scrub.

“During Monday’s launch attempt, preflight checks detected that a helium valve in the stage separation pneumatic system was not holding the right pressure. This meant that the stage separation pistons would be reliant on a backup check valve,” noted the company in a release on Wednesday.

“No issue was detected with the backup valve and a flight would likely have been successful, but SpaceX policy is not to launch with any known anomalies.”

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With the scrub called, the Falcon 9 v1.1 was detanked and then lowered via her strongback to allow for engineers to replace the valve and inspect the immediate system for additional confidence there won’t be a repeat issue during the next attempt.

“We have brought the vehicle back to horizontal and are replacing the faulty valve, as well as inspecting the whole system for anything that may have contributed to the valve not working as designed,” SpaceX added.

All of the work is expected to be complete well ahead of the next launch opportunity, which was – due to the orbital mechanics for Dragon heading to the ISS – no sooner than Friday.

In consultation with NASA, the Friday opportunity will now be taken, with a T-0 targeted for 3:25 pm ET. A launch on Friday results in a rendezvous with the Station on Sunday, April 20, ahead of being grappled at 7:14 am ET.

Notably, SpaceX’s discussions with NASA also opened up a slightly unexpected second opportunity on Saturday. The next two-day rendezvous opportunity was expected to be result in another four day gap before allowing Dragon to set sail. However, utilizing a three day rendezvous allows for a Saturday launch, should SpaceX require it.

Only a 40 percent chance of acceptable weather has been noted for the Friday attempt, meaning Saturday’s opportunity – which would occur at 3:02 pm local – could provide a welcome 24 hour scrub turnaround option.

If SpaceX launches Friday, a contingency spacewalk to replace a failed multiplexer-demultiplexer will be conducted on Wednesday, April 23. If SpaceX scrubs Friday and attempts to launch Saturday, the contingency spacewalk moves earlier to Sunday, April 20.

The specific MDM that failed on Friday night is the External-2 (EXT-2) MDM, which is a Tier 2 MDM responsible for controlling external US segment systems, and is one of two EXT MDMs located on the outside of the S0 Truss.

“EXT-2 MDM Failed to Start Up: Friday evening, EXT-2 MDM failed to bootup when ground controllers attempted to power it up to load a Pre-Positioned Load (PPL). A Total of five restart attempts were unsuccessful. In all cases the MDM was only drawing ~33 Watts when nominal in Wait state is ~50 Watts,” added notes from the MDM Special Section in L2.

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“The power to the MDM and 1553 bus channels both looked nominal. An ART (Anomaly Resolution Team) meeting was held Friday evening, and Team 4 and Next Worst Failure Teams were activated. Plans to get the spare MDM ready for installation (initiated).”

According to the L2 notes, a decision on the spacewalking duo opted to team up Expedition 39’s Steve Swanson as EV1, using EMU 3005, along with crew member Rick Mastracchio, as EV2, using EMU 3011.

“Status of EMU Suits: All three suits onboard (SN 3005, 3010, 3011) have all had the fan pump separator removed and replaced. All three suits have had two of three required water flushes,” added L2 Status. ” The final flush and Return to Service Checkout (has been) completed.”

The EVA, which is only expected to last 2.5 hours, will remove the faulty MDM from the S0 Truss, prior to a replacement taking its place. The fine details of this EVA are still being evaluated.

“Status of MDM Spare: Teams are working to address two topics with the spare before installation: 1) The Chotherm is too thin – interstitial material between the MDM and the heat pipe is too thin, and 2) the team is determining how to check out the spare MDM with the MDM On-Orbit Tester.”

Work was also completed over the weekend in staging the Mobile Transporter (MT) to support the upcoming EVA. All went to plan, including the first time use of new software associated with the translation.

“Mobile Transporter (MT) Translation: On Sunday the MT was Translated from WS4 to WS2 to provide access to the EXT-2 MDM worksite,” added L2 Status.

“Of note, this was the first MT translation utilizing new Umbilical Mating Assembly (UMA) automatic demate software included in EXT R8. The new software worked flawlessly and is designed to save a significant amount of commanding during MT translations.”

The notes also showed the large amount of work the ISS teams have undertaken to ensure the Station is in a good configuration prior to the EVA and Dragon’s arrival.

Most of this work relates to the next worst failure reconfiguration of ISS, with ground controllers reconfiguring many of the systems on the ISS, including contingency scenario’s where if the working EXT MDM were to fail, the ISS would still be ready for Dragon capture and berth.

(Images: via L2 Special Sections and NASA)

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